1. Technical Field
The present disclosure relates generally to a fuel cell system, and in particular, relates to a fuel cell system including a flat-type stack in which a plurality of the unit cells, in which an electricity generating reaction occurs, are arranged on a plane, and in which the fuel is preheated, thereby reducing the temperature differences of the fuel provided to each unit cell, and reducing the difference in the output characteristics of each unit cell, thereby increasing the electrical generating efficiency.
2. Description of the Related Art
A fuel cell is a power generation system that directly converts energy from a chemical reaction between a hydrogen-containing fuel, for example, a hydrocarbon-based material such as methanol, ethanol, and natural gas, and an oxidant into electrical energy. Examples of fuel cell systems include Polymer Electrolyte Membrane Fuel Cell (hereinafter, referred to as PEMFC) systems and a Direct Methanol Fuel Cell (hereinafter, referred to as DMFC) systems.
DMFC systems generate electricity by an electrochemical reaction between oxygen from an oxidizing agent and a methanol fuel provided directly to a stack. The DMFC system has a high energy density and a high power density, and since a liquid fuel such as a methanol or the like is used, DMFC systems advantageously do not need additional devices such as a reformer. Storage and supply of the fuel is also easy.
In the DMFC system, the stack that actually generates electricity has a structure in which at least one unit cell including a membrane-electrode assembly (hereinafter referred to as an “MEA”), and a separator or a bipolar plate are stacked. The MEA comprises an electrolyte membrane interposed between an anode electrode and a cathode electrode. Furthermore, each of the anode electrode and the cathode electrode is provided with a fuel diffusion layer for supplying and diffusing fuel to a catalyst layer, a catalyst layer where the oxidation/reduction reaction of fuel occurs, and an electrode support body.
Various kinds of the DMFC systems geometries may be formed in accordance with the method of supplying the air and the arrangement of the unit cells Some embodiments comprise a flat-stack type in which a plurality of unit cells is generally arranged on a plane. A flat-stack type fuel cell system is also referred to as a “passive” or a “semi-passive” type since it does not require a pump for providing the air.
A flat-stack type fuel cell system is typically formed in the shape of a plate with the plurality of unit cells arranged on a plane. Each unit cell comprises a cathode to which the air is supplied, and an anode to which the fuel is supplied. The cathode is supplied with the air by natural diffusion or convection of air, and the anode is supplied with fuel from outside the unit cell by a pump. Here, since the unit cells are arranged on a plane, the fuel is sequentially supplied from a unit cell on one side of the stack to a unit cell on the other side of the stack. Therefore, in the unit cell, the temperature of fuel in a unit cell depending on the position thereof. That is, in the flat stack, a unit cell arranged at the fuel supply opening receives fuel at a normal temperature, which is relatively low. However, the fuel is heated in the unit cell by the heat generated during the electricity generating reaction, and then flows to downstream unit cells in the flat stack. Therefore, a unit cell that is far from the fuel supply opening receives fuel at a relatively high temperature.
On the other hand, the electricity generating reaction that occurs in the unit cell is in turn affected by the temperature of the fuel. Therefore, in the flat-stack type fuel cell system, the output characteristics of the unit cell change depending on differences in the fuel temperature. In addition, the output and efficiency of a flat-stack type fuel cell system are reduced due to differences in the output characteristics of each unit cell.